Apparatus for neurosurgical stereotactic procedures

ABSTRACT

An apparatus and method are disclosed for displaying a path between a selected target and selected points on a patient&#39;s skull and for guiding surgical instruments along any selected path. The system is comprised of an image display system, an articulated arm and probe, and a stereotactic system. The sub-systems are coupled to one another so that the articulated probe may be used to select patient fiducial points that correspond to selected image fiducial points. Using these points, the image display system coregisters the external locations to the displayed images so that the probe condition may be displayed with the displayed images. The system further permits the identification of a selected target within a patient&#39;s brain and to project a path from the external position to the target prior to the performance of a craniotomy. After evaluation of the path, a surgeon may lock the stereotactic system in place to preserve a selected surgical path and to guide instruments along that path. A method of utilizing the system to perform such surgical procedures is also described.

FIELD OF THE INVENTION

This invention relates to neurosurgical apparatus generally, and moreparticularly, to stereotactic systems for use in neurosurgery.

BACKGROUND OF THE INVENTION

During the 1970's radiological imaging systems were developed to assistsurgeons in ascertaining the internal condition of a patient in greaterdetail. Specifically, computer assisted tomography (CAT) systems weredeveloped to enhance images generated from data produced during aradiological scan of a patient. The patient is placed within a gantry,and a radiation source and radiation detectors are positioned oppositeone another to be rotated about a portion of the patient's body. Thedata generated by the radiation detectors are utilized by a computer togenerate radiographic images or "slices" of the body position to give adoctor greatly enhanced views through the area of interest.

Later radiographic imaging systems included magnetic resonance (MRI) andpositron emission tomography (PET) imaging which generate images fromenergy sources that do not use x-rays or the like. These devices areuseful because they provide different or additional information aboutorgans or tissues than CAT scan images. In this application the termscanners refers to imaging devices regardless of the technique utilizedto generate the images.

Neurosurgery may be performed to investigate, repair, or removeanomalies located within the brain of a patient. The environment of suchsurgeries is challenging in that the organ of interest, the brain, issurrounded by relatively thick bony structure, the skull. The onlypresurgery access to the brain available to a surgeon is through imagesgenerated by an imaging system.

Because of the inaccessibility, size, and roughly hemispherical shape ofthe brain, specifying the locus of a point inside the brain generallyrequires reference to some fixed external reference system. To provide asurgeon with sufficient information to locate an area of interest on animage, such as a tumor or lesion, a variety of systems have beendeveloped to provide a reference point or points which may be used tomatch the patient's anatomical structure with the structures displayedin the images. These systems typically require that a frame be rigidlyfixed to a patient's head to provide a reference point or points. Oncethe reference structure is attached to the patient, the image data isgenerated with the reference frame fixed in relation to the imagingdevice, That is, there is typically a mechanical coupling between thereference structure and the imaging device, After the data is collected,the patient may be removed from the scanner but the reference frame mustremain attached to the patient's head. The reference frame remainsattached throughout surgery so the surgeon can correlate imageinformation about patient anatomical structures to a position within thepatient's skull located with reference to the frame.

While such systems provide surgeons with a remarkable ability to locateareas of interest within a patient's brain based upon the data acquiredby radiological scanners, the required reference frames are cumbersomeand complicate the acquisition of radiological data. To preserve thelocation of the reference frame, it must remain attached to thepatient's head throughout the scanning procedure and the surgicalprocedure. Because the reference frames may weigh several pounds andmust be securely fastened to the head, they can be uncomfortable to thepatient. The distances the frames extend from the patient's head alsopresent difficulties in maneuvering the patient. Additionally, patientswith larger than normal heads often cannot be fitted with stereotacticframes.

In an effort to reduce the awkwardness of the reference structure andthe discomfort it causes a patient, a stereotactic system using a skullring which may be mounted to a patient's skull was developed. The ringis a relatively small metallic circle that is attached to a patient'shead using cancellous screws. Once the ring is in place, a transferplate having two openings, one of which has a rotatable ball and socketmechanism mounted therein, is secured within the ring. The transferplate is also provided with a radiological opaque marker which may bediscerned in the radiological images generated by the scanner. Thepatient is then placed inside a scanner and a member extending from theball and socket is coupled to the machine. Once the patient has beenoriented within the scanner for the collection of image data, the balland socket is locked in a fixed orientation.

Following the collection of image data, the member extending from thering and patient which was coupled to the scanner is disconnected so thepatient may be removed. The ball and socket remains locked in itsorientation so the orientation of the transfer ring on the patient'sskull may be later duplicated for locating a target.

After removing the transfer plate holding the ball and socket from theskull ring attached to the patient's head, the plate is attached to amember extending above a frame table to duplicate its position andorientation on the patient's head. The images generated by the scannerare viewed and the coordinate data of a selected target, such as alesion or tumor, and the radiological marker of the transfer plate aredetermined. Using this coordinate data and the indicia marked on theframe table, a target marker is maneuvered on the frame table so itidentifies the target position with respect to the radiological marker.A second ball and socket mechanism is placed in the second opening ofthe transfer plate. Thereafter, an instrument such as a biopsy probe maythen be extended through the second ball and socket to the target pointto define a distance and path to the target. The second ball and socketis then locked into place to preserve the orientation to the target andthe distance to the target is marked on the probe.

The transfer plate bearing the second ball and socket mechanism may thenbe removed from the member above the frame table and reattached to theskull ring on the patient's skull with the second locked ball and socketdefining a path to the selected target. Thereafter, a biopsy probe maybe used to mark the patient's skull and a craniotomy performed at thatpoint to provide an opening in the patient's skull. The biopsy probe maythen be extended through the opening in the second ball and socket tothe depth marked on the probe to place the biopsy probe within thelesion or tumor. In this manner, the surgeon is able to accurately placethe biopsy probe without unnecessary searching to locate the tumor orlesion prior to performing the biopsy. A further description of theabove technique and apparatus is given in U.S. Pat. Nos. 4,805,615 and4,955,891 to which reference may be had.

The above-described manner for performing the biopsy facilitates thecollection of image data in a number of ways. First, the referencestructure attached to the patient's skull is small in comparison to thereference frames previously used. Second, the removable plate with theball and socket openings permit accurate location of a target areawithin a patient's brain prior to performing a craniotomy. Third, theremovable plate with the ball and socket mechanisms ensures correctplacement of the plate on the patient's skull and preserves the accuracyof the path to the target identified on the frame table. While thismethod greatly facilitates locating the target area within a brain, itfails to provide the surgeon with information regarding the interveningtissue area between the craniotomy opening in the skull and the targetarea, which lies within and possibly deeply within the brain.Furthermore, the image data generated by a scanner is not necessarilyoriented transversely to the location of the opening of the ball andsocket of the reference ring and thus does not provide image data atvarious depths between the craniotomy opening and the target area toassist the surgeon in evaluating the path to the target. Thus, while thesurgeon need not search to locate the target, the surgeon does need tocarefully retract the brain tissue along the path to reach the target.Otherwise, damage to any sensitive areas that may lie along the pathwayis possible. The reference systems discussed above do not assist asurgeon in identifying the exact location of any such sensitive areasprior to performing the craniotomy and traversing the path to thetarget.

In addition to identifying the locus of the lesion or injury within thebrain it is often critical to determine a suitable pathway through thebrain to access that locus, in order to minimize damage to theintervening tissue. Thus, identifying the pathway to the site may bealmost as critical as identifying the site itself. The above-describedsystem has been inadequate in this respect.

In an effort to provide more automatic matching between image data andthe patient as placed in surgery, systems have been developed thatperform "coregistration". Coregistration is a process by which acomputer matches fiducials associated with image data to fiducialsassociated with the patient's body. The image fiducials are typicallyselected by using a mouse and cursor to identify on a displayed imagepoints that lie on a patient's skin. An articulated arm and probe arecoupled to the computer to provide coordinate data for points externalto the computer. Using the arm and probe, the user selects points on thepatient that correspond to the selected image fiducials and the computerexecutes a program that matches the corresponding points. After asufficient number of points have been selected (usually at least 8), thecomputer may identify the point in the displayed images that correspondsto the position of the probe proximate the patient's head. Such a systemis made by Radionics of Brookline, Mass. and is identified by itsproduct name The Operating Arm.

Such a system provides "navigational" information to a surgeon, that is,the surgeon may bring the probe to a particular location on or within apatient's head and have that location identified on the displayed image.In this way, the surgeon may view areas on the displayed image anddetermine their proximity to the probe location. In that manner, thesurgeon may confirm the surgical approach to a target.

While these systems provide confirming navigational information theystill do not project a stabilized image of the surgical path on adisplayed radiological image prior to a craniotomy being performed. Suchsystems cannot project a stabilized path because the surgeon cannotconsistently orient and stabilize the probe at exactly the same positioneach time the path needs to be viewed. As a consequence, such systems donot identify or persistently indicate a path to a target because theprobe is operated by hand. Moreover, such systems do not ensure that thesurgeon is following any path the surgeon may have selected as a resultof viewing the displayed radiological images.

What is needed is a system that permits a surgeon to select, evaluate,and lock into position a path to a selected target prior to performing acraniotomy. What is needed is a system that guides a surgeon along theevaluated surgical path to a target during and after the craniotomy.What is needed is a way to select and preserve a plurality of selectedpaths to multiple targets after the paths have been evaluated.

SUMMARY OF THE INVENTION

These and other problems of previously known systems are overcome by asystem in accordance with the principles of the present invention. Thissystem includes an imaging display system for displaying radiologicalimages, an image fiducial selector coupled to the imaging system forselecting fiducials on an image displayed on the display system, atarget selector coupled to the imaging system for selecting a target onan image displayed on the display system, an articulated arm and probecoupled to the imaging system, which provides spatial coordinates forthe probe with reference to the imaging system so that a positionassociated with the probe is displayed on the displayed image. A patientfiducial selector is coupled to the imaging system and to thearticulated arm for selecting fiducials on a patient that correspond tothe fiducials selected for the displayed image. A coregistrationprocessor coregisters the patient fiducials to the selected imagefiducials so that the coordinates provided by the articulated arm may bematched to the displayed image whereby a position of the probe may bedisplayed on the displayed image. A probe holder holds the probe of thearticulated arm in proximity to a patient's head, the holder beingselectively lockable to maintain a position proximate the patient'shead. By using this system, a surgeon may evaluate the path displayed onsaid displayed image between said probe position and said selectedtarget.

A system in accordance with the principles of the present inventionpermits a patient to be scanned without any plate or frame referencebeing affixed to the patient. The system coregisters image fiducialswith selected anatomical features of a patient so the position of theprobe may be displayed on a radiological image and a path to a selectedtarget projected on the image. A surgeon may evaluate the path to theselected target and lock the probe position in place if the path isdeemed acceptable. The surgeon may then mark the appropriate spot on thepatient's head for the craniotomy. In a similar manner, the paths toother targets may be identified and marked prior to any craniotomy.

The system may further include surgical instrument collars adapted tofit within the probe holder so an instrument may be inserted through thecollar in the correct orientation and position to follow the evaluatedpath to the selected target. Thus, the probe holder may be used tofacilitate a surgeon's path selection and evaluation and then preservethat path as well as guide instruments along that path.

The system of the present invention may further include an arc carrierrod for defining a predetermined radius to a selected target. A groovedarc may be rotatably mounted to the reference rod, and the probe holdermounted in a probe adapted to slide within the grooved arc. Thus, thegrooved arc may be rotated in a hemispheric fashion about the patient'shead and the probe plate and holder slid along the grooved arc to definenumerous entry points for evaluation by the surgeon using theradiological display system. The imaging system is further provided witha processor for interpolating data from the radiological data generatedby the scanner to provide a view along the probe from any entry portselected along the hemispheric stereotactic system positioning as longas the probe reaches the holder. Utilizing this system, a surgeon mayevaluate a number of entry points and select the one which presents theleast risk to the patient.

Another advantage of the present system is that after a target has beenselected and the biopsy or surgical procedure performed on the target,the surgeon may select a second target of interest within the patient'sbrain. After this selection, the probe holder may be unlocked and theprobe reinserted to define a second path to the second selected target.The hemispheric stereotactic system may then be attached to providemultiple entry points to the second target for evaluation and, once asuitable path is selected, a procedure may be performed on the secondtarget. Utilizing the system in this manner facilitates a surgerywherein radioactive seeds are implanted in various areas of a tumor withthe effect that the radiation is primarily limited to the area of thetumor. This type of use also assists a surgeon in the precise placementof multiple depth electrodes in a patient's brain for monitoring.

These and other advantages of a system in accordance with the principlesof the present invention may be ascertained with reference to theattached drawings and enclosed detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention may take form in various components andarrangement of components and in various steps and arrangement of steps.The drawings are only for purposes of illustrating a preferredembodiment and alternative embodiments and are not to be construed aslimiting the invention.

FIG. 1 is a perspective diagrammatic view of the components of oneembodiment of a system in accordance with the principles of the presentinvention;

FIG. 2 shows a representative screen displaying image informationgenerated by the system of FIG. 1; and

FIG. 3 is a view of the preferred embodiment of a stereotacticsub-system for use in the system of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

A neurosurgical stereotactic system 10 built in accordance with theprinciples of the present invention is shown in FIG. 1. The systemincludes an image display sub-system 12, an articulated arm and probe18, and a stereotactic sub-system 16. The image display sub-system 12displays images from image data generated by a scanner or from datainterpolated from such data. Sub-system 12 accepts operator input forselection of fiducials, receives coordinate data from the articulatedarm and probe, and coregisters selected fiducials on a patient 13 withthe selected fiducials for the radiological images for the patient sothat the position of the probe and a path to a selected target may bedisplayed. Sub-system 12 also displays an image of the articulated armso the operation of the arm and probe may be verified.

Articulated arm and probe 18 provide spatial data to display sub-system12 through an encoder interface 20. The spatial data is preferablygenerated by optical encoders 22, although other spatial coordinate datagenerating components may be used. Besides the data supplied by the armand probe 18 used to locate the probe's position, probe 24 may alsosupply rotational data as it is rotated about its longitudinal axis torotate the displayed image on sub-system 12, as described in more detailbelow.

Stereotactic sub-system 16 stabilizes the probe 24 as a surgeon guidesit across a patient's head. Sub-system 16 further includes components,discussed in more detail below, that permit the probe to be locked intoposition, and that position utilized to guide surgical instruments to aselected target. Sub-system 16 further includes components, alsodiscussed in more detail below, that may be used to provide multipleentry points for a surgical path to a target within the patient, all ofwhich are centered on the selected target area. These components providea surgeon with reasonable confidence that each probe position providedby the system is directed to the selected target.

Radiological display subsystem 12 includes a computer 30 to which a highresolution graphics monitor 32, a mouse 34, a footpedal 36, a keyboard38 and a tape drive 40 are coupled. The computer 12 may additionallyinclude a 3.5 inch diskette drive or the like and a digital audio tape(DAT) drive or the like. The tape drive 40 diskette drive, and DAT drivemay be used to provide radiological imaging data to the computer 30.These tape drives may also be used to archive data generated by thecomputer 30 or to update the software which executes on the computer 30.Computer 30 may also be coupled using conventional techniques to acomputer network such as an Ethernet. Such a network may be used tosupply radiological image data, software, or diagnostic services.

Preferably, monitor 32 is a Multi-ScanHG Trinitron superfine pitchresolution monitor available from Sony Corporation of America.Preferably, the computer 30 is a Dell 450 DE/2 DGX manufactured by DellComputers of Houston, Tex. The preferred tape drive 40 for reading imagescan data is a 9 track tape drive manufactured by Overland Data of SanDiego, Calif. The encoder interface 20 and articulated arm and probe 18are manufactured by Immersion Human Interface Corp. of San Francisco,Calif.

Preferably, computer 30 executes the Atlas program developed by Nomos ofPittsburgh, Pa. Atlas is a computer program that displays radiologicalimages from radiological scan data supplied by the tapes andinterpolates data to provide additional views not present in theradiological scan data. The Atlas program of the preferred embodimenthas been modified to accept data from the articulated arm and probe 18through encoder interface 20. The program is loaded by using theresident operating system of computer 30 which in the preferredembodiment is the Microsoft Disk Operating System (MS-DOS). The Atlasprogram includes its own high level I/O routines and other computerresource functions so that the Atlas program uses the primitive levelI/O operation of the resident operating system of computer 30. In thepreferred embodiment, computer 30 is also provided with a telephoneinterface so that software and other support functions, such asdiagnostics, may be provided via telephone from a remote location.

The articulated arm and probe 18 are mounted to a surgical skull clamp46 which has been mounted to an operating table 48 (which may be ofknown type). Base support 50 (FIG. 1) is attached to a mounting collar52 which is mounted to the starburst connector 54 of surgical skullclamp 46. Base support 50 is preferably mounted to collar 54 by Allenscrews or the like. Preferably, the mating surfaces of collar 52 andsupport 50 are keyed at 53 so there is only one possible orientation ofthe base support 50. This feature is important in preserving referencepoint accuracy when the sterile base support and surgically draped armare used as discussed in more detail below. Base support 50 alsoincludes a lockable mounting bolt 56 at one end for the articulated armand a hollow tubular extension 58 at its second end for holding in bore59 the probe 24 of the articulated arm. Bolt 56 is rotatably mountedabout a slot 60 cut in base support 50 for articulated arm and probe 18.

The articulated arm and probe 18 (FIG. 1) further includes a mountingstud 62, two arm members 64, and the probe 24. Joint members 68 joinmounting stud 62, arm members 64, and probe 24 to form arm and probe 18.At each joint, there is rotation in two perpendicular planes to permittwo degrees of freedom for each arm. The position of each arm memberrelative to its respective joint is preferably provided by opticalencoders 22 coupled at each joint to the arm in an orthogonalrelationship. Probe 24 is mounted within a collar 70 located at theoutermost end of the arm so that it can rotate about its longitudinalaxis. This rotational movement is used by computer 30 to rotate theradiographic images presented to the surgeon on the screen of monitor32. Extending from one end of the articulating arm 18 is an interfacecable 72 which terminates at an encoder interface 20. The encoderinterface 20 converts the data from the six optical encoders 22 of thearticulating arm 18 into rotated position (angular) data for thecomputer 30.

Tape drive 40 may be used to provide image scan data to the computer 30.Most image scanners archive image data generated from a scan by storingit on magnetic media such as a nine track tape 74. This tape may then beread by a tape drive 40 and supplied to the computer 30 which stores thedata on other magnetic media such as a hard disk drive. The image dataread from the tape inserted in drive 40 may be used as generated by thescanner. However, each scanner manufacturer may format the datadifferently. Preferably, the image data generated by the various typesof scanners is converted to a standard format prior to being stored onthe internal magnetic media of the computer 30. By doing so, the imagedisplay program which executes on computer 30 does not require differentmodules or routines for each format in order to utilize the data fromvarious scanners.

Generally, data generated by a scanner includes image data and non-imagedata. Non-image data includes definition of parameters such as patientname, date, patient position, scan orientation, scan parameters, andother imaging details peculiar to each of the various scannermanufacturers. The preferred embodiment of the program executing oncomputer 30 extracts the basic data items common to all of the scannermanufactures and stores them with image data files in a keyword valuefile. The keyword value file contains a list of keywords that identifyeach data field and the value of that field. For example, a data fieldidentifier for patient name is followed by the data representation ofthe patient's name for a series scan, These files are preferably humanreadable for system analysis purposes since they are not usuallyaccessed by a user.

Image data usually includes numerical data that represents a gray scalevalue or some other brightness/contrast value, such as Hounsfield units,used to generate images, as is well known. These numeric values may becompressed, or expressed as integer or real number values. The preferredembodiment of the program executing on computer 30 uncompresses anycompressed values and converts all of the numeric data to integer data.This data is then stored in image data files. These files are preferablywritten to disk in a hierarchial structure separating the patient datafrom one another and the image studies and series for each patient.

The footpedal 36, mouse 34, and keyboard 38 may be used by an operatorto provide input to the computer 30. For example, mouse 34 may be usedto manipulate a cursor on the screen of monitor 32 to select variousoptions as discussed in more detail below. As a further example,footpedal 36 may be used by the surgeon to activate the selection offiducials on a patient.

In the preferred embodiment, the image display program executing incomputer 30 includes a graphics user interface (GUI), an input/output(I/O) library, an articulated arm interface program, and a number ofapplication modules. The GUI interface controls the presentation of dataand menus on the screen of the monitor 32. The I/O library routinesperform various input and output functions such as reading image datafrom the tape drive 40. The articulated arm interface provides the menuand fiducial selection points displayed at the bottom of the screen onthe monitor 32 of the preferred embodiment of the sub-system 12 shown inFIG. 1. Finally, the application modules execute software to performtransform operations to interpolate data for the images and tocoregister the image data with the selected patient fiducials.

An alternative embodiment of stereotactic sub-system 16 that couples thearticulated arm and probe 18 to the patient to permit surgical pathevaluation and selection is shown in FIG. 1. That equipment includes askull ring 80, a transfer plate 82, a swivel socket 84, and a probealignment ball 86. This equipment is utilized by affixing the skull ring80 to a patient's head by cancellous bone screws after the patient'sscalp is shaved, prepped with betadine, and injected with xylocaine.After the skull ring is affixed, the transfer plate 82 is mounted to theskull ring by means of a post (not shown) extending from the skull ring.A swivel socket 84 is attached to the skull ring by means of Allenscrews or the like. The swivel socket 84 includes a base 92 and aupwardly extending collar 94. A probe alignment ball 86 is insertedwithin collar 94. The probe alignment ball 86 is adapted to receive theend of probe 24. Thus, probe 24 may be inserted into the probe alignmentball 86 and the probe and ball moved together with respect to thesurface of the patient's scalp.

Once a particular orientation has been selected by the surgeon basedupon information provided by the radiological image displayed on monitor32, the screws 98 extending outwardly from the collar 94 may betightened to secure the probe alignment ball 86 in place. A surgicalinstrument collar of known type (not shown) may then be inserted withinthe probe alignment ball 86 to permit a drill or other instrument 87(shown in phantom) to be inserted through the instrument collar to openthe patient's skull. A biopsy instrument may also be inserted throughthe collar to the target area. Thus, use of the ring 80, transfer plate82, socket 84, and ball 86 provide a surgeon with a stable platform fororienting probe 24 and securely locks an evaluated orientation in placeto provide a guide for the surgical procedure.

The hemispheric stereotactic system used for entry site selection isalso shown in FIG. 1. That equipment includes an arc carrier rod 100, arotating support arm 102, an arc 104, and a variable collar array 106.After the probe alignment ball 86 has been oriented so probe 24 pointsto the target, the probe may be removed and the arc carrier rod 100inserted into the probe alignment ball 86. The rotating support arm 102is then mounted on the arc carrier rod 100 and secured about the rod byscrew 110. A grooved tongue or key 112 is mounted in lockablerelationship on the rotating support arm 102 and is adapted to fitwithin a track cut within arc 104, By tightening screw 118 of therotating supporting arm 102, the arc 104 may be secured to arm 102anywhere along the length of arc 104. Variable collar array 106 islikewise adapted to have a bit that is slidably received in arc 104 andmay be locked into place anywhere along the length of arc 104. Collar106 also has a receptacle that receives the probe 24 so a surgeon mayevaluate a path to the selected target by viewing the path displayed onthe monitor 32 of sub-system 12. Because carrier rod 100 points to thetarget, the support arm 102 and arc 104 may be rotated about thepatient's head in a hemispheric fashion that is centered about thetarget. Preferably, support arm 102 is locked into position about thearc carrier rod 100 so that a central opening in the variable collararray 106 is located approximately 19 centimeters from the target aboutwhich the arc 106 is centered.

The components of the hemispheric stereotactic system permit a surgeonto maneuver the probe 24 about a patient's head with a reasonable degreeof confidence that the receptacle is directed to the previously selectedtarget. By simply swinging the arc 104 around rod 100 and sliding thecollar 106 within arc 104, the surgeon is provided with numerous sitesfor evaluation which may be locked in place as a surgical guide.

The preferred embodiment of stereotactic system 16' is shown in FIG. 3.That system includes a probe holder collar 180 and a rigid offset arm182 which is interposed between sunburst connector 54 and collar 52.Offset arm 182 terminates in a pivot joint 184 from which an adjustablearm 186 extends. Another adjustable arm 190 extends from a pivot joint190 at the end of arm 186. Arm 188 terminates in to a probe holder 192which is provided with a transfer plate 82, ball and socket mechanism 84and adjustment ball 86, as already described in connection with FIG. 1.Thus, sub-system 16' provides a rigid, adjustable arm by which the probeholder 192 and attendant components may be maneuvered about a patient'shead and then selectively locked into position for path evaluation,surgical instrument guidance, or attachment of the hemispheric system.

To use the system 10 for a neurosurgery, a patient 13 is scanned in animage scanner to create a series of images. A "series" may be a group ofparallel, equally spaced images, sometimes called "slices", of avolumetric portion of a patient's body. Preferably, the imagescomprising the series are contiguous. A group of more than one series ofimages is commonly referred to as a "study" or "suite" and may also beutilized by the system. Examples of series are axial, coronal, rotating,and sagittal. An axial series is from the top of the patient's head tothe base of the skull, a coronal series is from the face to the back ofthe patient's head, rotating series is around a patient's head, and asagittal series are sideviews of the patient's head to the other side.Preferably, the series is generated with a gantry angle of 0°, otherwisethe data interpolation performed by the preferred embodiment of theAtlas program may be distorted.

After the series is generated by the scanner, it may be written tomagnetic media for transportation to system 10, Typically, the imagedata is written to a nine track tape 74 which may be read by the ninetrack magnetic tape reader 40. The user may activate computer 30 andactivate the nine track tape interface program in the I/O library. Byusing this program, a user may read the image data from the nine tracktape 74 into the computer 40 which then stores the data in anappropriate format to a hard drive or the like. Computer 30 may alsoreceive data from an image scanner by means of the DAT reader, diskettedrive, a computer network, or the like.

After the image data is read into the computer's memory, the user mayexecute the program which displays the image radiological data in adisplay window on the monitor 32. The user may select a particular viewby clicking on the view icon 120 of the screen shown in FIG. 2. Thataction causes a menu to be displayed for various series or views fromwhich the user may select. After a series has been selected, the firstimage or slice of the series is presented in the display window 122 asshown in FIG. 2. The user may view each of the images in the series bymanipulating the slider 124 button on the screen with the mouse 34.

The user may select a second series to be displayed in a second displaywindow 126. After creating the second display window by clicking on theview icon 128 for the second window, the user may select the particularseries to be displayed in that window and likewise view the variousslices by manipulating the slider button 130 with the mouse 34.

Computer 30 also may generate a second series that was not generated bythe scanner. The computer 30 does this by interpolating data from one ofthe series generated by the scanner to generate the second series. Forexample, a coronal series may be generated by the scanner and displayedin the first display window of the system. If the user selected asagittal series, which was not generated by the scanner, for display inthe second window, the system interpolates the data at the righthandedge at each of the coronal images and generates a sagittal view fromthat data. This process is repeated for equally spaced, parallel imagesfrom the coronal perspective to create the second saggital series.

In the preferred embodiment, the Atlas program reformats the image datato generate data that represents a volumetric representation of thescanned area. This is done by interpolating the image data for theindividual slices to generate additional "slices" not acquired by thescanner. Preferably, this is done by generating so-called "voxel" valuesthat represent an image value that is cubic in dimension, although othervolumetric shapes may be used. For example, if the series is made ofimages that represent 3 mm slices and each pixel value represents a 0.5mm×0.5 mm, the interpolated voxels preferably represent a cube which is1 mm×1 mm×1 mm. To interpolate the voxel values for the voxels in theplane of the image, each group of four adjacent pixels forming a squareare averaged and the resulting averages comprise the data to form the inplane image of 1 mm×1 mm×1 mm voxels. For the voxels that represent theplanes at the 2 mm and 3 mm depths, the inplane voxel image ispreferably combined with the underlying image plane (reference plane) atthe next lower 4 mm plane using a linear weighing proportional to thedistance from the selected plane to the reference plane. Of course,other interpolation schemes may be used as are well known in the art.After the interpolated data is generated, Atlas uses the data togenerate any series requested by a user.

Once the display window or windows are created and an appropriate imageseries displayed within those windows, the user may select imagefiducial points. This is done by clicking the mouse on the appropriateareas within the image fiducial point identifier menu as shown in FIG.2. After activating one of the image point icons 140, the user may usethe mouse 34 to manipulate a cross-hair cursor across the image andafter centering it on a particular feature, clicking on the mouse buttonto cause the program executing in computer 30 to match the point on theimage with the selected image point icon 132. The user may do this for,say, eight points, although fewer or more points may be implemented in asystem in accordance with the principles of the present invention. Atleast three points are needed before coregistration may occur and, mostpreferably, coregistration is best achieved with approximately eight toten points.

The mounting collar 52, base support 50, and articulated arm and probe18 are mounted to the skull clamp 46 holding the patient's head.Preferably, the skull clamp 46 is one manufactured by Ohio MedicalInstrument Co., Inc. of Cincinnati, Ohio and designated as a modifiedMAYFIELD® clamp. The clamp includes a ratchet arm 146 mounted within asleeve arm 148 and further includes a two pin bracket (not shown)mounted to the sleeve arm and a torque screw and pin 150 mounted to theratchet arm 146. This clamp is adjusted to fit the patient's head bywell known procedures.

Located on the sleeve arm is a starburst connector 54 to which themounting collar 52 is mounted. Base support 50 for the articulated armand probe 18 is attached to the mounting collar 52 by means of Allenscrews or the like. The articulated arm and probe 18 are attached to thebase support 50 as discussed above and the probe 24 is placed in thetubular extension 58 of the base support in preparation for probeinitialization (shown in phantom in FIG. 1).

In the preferred embodiment, the user must place the articulated arm sothat all the side mounted optical encoders are on the same side of eacharm segment. If the arm is placed in an incorrect position, the computer30 and encoder interface 20 interpret the angular data from thearticulated arm as being in a direction opposite that of its actualmovement and improperly display the probe's position. Once the arm isthe appropriate location, the probe may then be initialized.

By clicking on the probe initialization icon 154 (FIG. 2), the userpermits the computer 30 to begin accepting angular data input from thearticulated arm and probe 18 and to initialize the probe's positionwithin extension 58 as a reference point. A display area 156 is shown onthe lower left portion of the screen on the monitor 32 whichdemonstrates the position of each arm segment and the tip of the probe24. By retracting the probe 24 from the tubular extension 58, the usermay manipulate the articulated arm and probe tip in space and observeits movement on the screen. In this way, the user can verify that theoptical encoders 22 were in the correct position for initialization bynoting the upward movement of the probe on the screen when the probe ismoved in an upwardly vertical position, for example. If the displayedprobe moves in a direction opposite that in which the probe tip isactually moved, then the user knows that the arm was initializedincorrectly and should repeat initialization with the articulated arm inthe proper position.

After confirming that the probe was properly initialized, the user maythen place the probe tip on the external points of the patient's skullthat correspond to the image fiducials previously selected. Typically,these points include the bridge of the nose aligned with the center ofthe eye sockets or the like. This is done by having the user first clickon the select button adjacent a patient point icon 158 and then placingthe probe 24 at a point on the patient's skull that corresponds to theimage fiducial associated with the activated patient point. Bydepressing the footpedal 36, the coordinates of the patient fiducialidentified by the probe's position are associated with the activatedpatient point. The reader should note that the selection of the patientfiducials may precede the selection of the image fiducials.

After the user has selected at least three patient and image fiducialpoints, computer 30 begins to execute a program to coregister theradiological display data with the selected patient fiducials.Preferably, the program implements an iterative algorithm for performingthe coregistration. An indicator window 160 is provided on the screen ofmonitor 32 (FIG. 2) to provide the user with information regarding thequality of the coregistration between the radiological data and theselected patient fiducial points. Typically, the coregistration improveswith the number of selected points and the number of approximately eightto ten points normally provides excellent registration between thepatient and the image data.

Preferably, coregistration is preformed by an iterative algorithmimplemented in one of the program modules executing on computer 30. Thepreferred algorithm selects a set of image fiducial points and thecorresponding patient fiducial points. The centroids of the geometricfigures defined by each selected set are computed. The coordinates ofone of the centroids are then translated to the coordinates of thesecond centroid and the points associated with the first centroid arelikewise translated. The differences in coordinates of the translatedand untranslated points for the first set are squared to determine anoverall error value or merit figure. The translated points are thenchanged an incremental amount in one direction only and the differencebetween the points in the first set and their corresponding points inthe second set are calculated and squared. If the error result is lessthan the merit figure then the incremented value becomes the pointvalues for the first set and the error result becomes the new meritfigure.

The incremental change is again performed in the same direction and anew error result calculated. When the error result is greater than thecurrent merit figure, the translated points are deemed the best fit.

The incremental change now is computed and evaluated for anotherdirection. Incremental changes in the second direction continue untilthe error result is greater than the merit figure and the previouslytranslated point deemed the best fit. The incremental changes thencontinue in the previous direction until the error result is greaterthan the current merit figure. The incremental changes again are testedfor the second direction. This process continues until no translation ineither direction generates an error result less than the current meritfigure. When this occurs, the third direction is incrementally changedand tested using the error result and merit figure computed as describedabove. When no incremental change in any direction produces an errorresult greater than the current merit figure, the coregistration iscomplete and translation of all points in one set to correspondingcoordinates in the second set may be performed.

Sometimes a patient fiducial does not correspond accurately with thepoint selected in the image data and may degrade the rating of thecoregistration. The deactivation of such a point, by resetting thecorresponding "use" icon, may improve the coregistration rating.Computer 30 permits a user to selectively activate and deactivate pointsto determine which points provide the best registration between thepatient and the image data. The reader should note that coregistrationimproves with the number of fiducial points, however, so the betteraction is to reselect one of the image or patient fiducials so it bettercorresponds to its mate.

Either prior to the coregistration or following it, the user may selecta target. This is done by activating the set target icon 162 on thescreen of monitor 32 with the mouse 34 and moving cursor cross-hairswith the mouse to an area within the interior of the patient's headdisplayed within the image. Typically, the area of interest may be atumor, a lesion, a suspected point of activity for which a surgeonwishes to place a depth electrode, or an area for the implantation of aradioactive "seed" for radiosurgery. The target coordinates areestablished by clicking on the set target icon 162.

After coregistration and target selection have been performed, the usermay the place the probe 24 anywhere on the patient's skull (so long asthat area was part of the area scanned in the scanner), and a path 170is displayed from that point (marked by the arrowhead 172) to the target(marked by the cross-hairs 174). The path to the target is preferablydisplayed to indicate whether the path is actually present on the imageslice being displayed at that time. For each position, the distancebetween the probe position and the selected target along the displayedpath is shown in the lower right corner of the display window (FIG. 2).For example, a surgeon may place the probe at a point which is shown onan image slice presently being displayed. However, the surgeon may beholding the probe at an orientation such that the path from the probe 24to the target would traverse one or more slices prior to arriving at thetarget. In that case, the area actually present on the slice beingdisplayed is preferably shown in yellow and the remainder of the path tothe target is preferably shown in red (FIG. 2). By manipulating theslider button 130 with the mouse 34, the surgeon may view each portionof the target path in the appropriate image slice until the target isreached to evaluate the tissue that the selected path traverses. If thesurgeon decides that the path presents risks that are unacceptable, thesurgeon may select a different orientation or different point on thepatient's skull and re-examine the path.

By selecting the view "across the probe", a series is shown in thesecond display window which may be manipulated by using the mouse 34 tomove the slider button 130 for that display window. In this manner, theimages of the planes transverse to the probe 24 are shown from the probe24 to the target area. In this way, the surgeon may evaluate the paththrough the patient's brain tissue to get to the target.

Once the surgeon has selected a particular path to the selected target,a skull ring 80 may be attached to the patient's head and a transferplate 82 installed on the skull ring 80. The transfer plate 82 includesa probe alignment ball 86 in which the probe 24 may be located. Afterthe probe 24 is placed within the ball 86, it may be moved to againselect the path to the target area which is optimal in the surgeon'sopinion. While holding the probe 24 at that location, the ball 86 may belocked into position to define a path to target and the probe removed.Preferably, the apparatus shown in FIG. 3 is used to avoid attachingskull ring 80 to the patient's head.

Preferably, a second support base 50 and probe 24 that may be sterilizedreplace the support base and probe used for fiducial identification andcoregistration. The support base and probe that may be sterilized ispreferably made from gray anodized aluminum and the non-sterilized baseand probe are made from black anodized aluminum. Preferably, thesterilizable version of the tubular extension 58 and probe 24 areshorter than the non-sterilizable counter-parts to facilitatesterilization. However, the mounting collar 70 of probe 24 attaches tothe probe mounting stud extending from the outermost joint of thearticulated arm at a second location to ensure the reference point usedfor coregistration is not disturbed.

The sterilizable base support is mounted to collar 52 after a sterilizedsurgical drape with an opening cut therein is placed over the collar 52.Prior to mounting articulated arm and probe 18 to the base support 50, asterilized tubular drape is placed over the arm and mounting stud 62.Probe mounting collar 70 is then placed over the tubular drape to affixthe sterilizable probe 24 in place. The probe is then returned to theextension 58 and the user clicks on the gray probe icon 176 (FIG. 2) sothe Atlas program may adjust for the length of the probe 24. Thethickness of the surgical drapes are compensated in the machinery of thesterilization parts. That is, the opening in the mounting collar isenlarged an appropriate amount to compensate for the surgical drapebetween the mounting post and collar. This compensation is required toensure that the screws used to lock the probe to the post do not pullthe probe tip off of the center line of the post. The user then clickson the initialization probe icon 154 to verify arm placement.Coregistration is automatic using the previously selected fiducials.

At this point, the surgeon may use an appropriate drill to open thepatient's skull and insert an instrument collar into the transfer plate82. The surgeon could then insert, for example, a biopsy needle directlyto the target at the indicated depth and be reasonably confident thatthe biopsy needle is at the target area having passed through only thetissue viewed in the images displayed in the display window. Followingthe procedure, the surgeon may close the opening or further treat thetarget area. Likewise, the surgeon may select other targets and treatthem as the first one was.

After mounting the skull plate 80 and locking the ball 86 of plate 82into position, the surgeon may decide to mount an arc rod to provide ahemispheric instrument positioning system that is centered about thetarget area. This is done by attaching the arc rod 100 to the ball 86,mounting the rotating support arm 102 to the rod 100 and attaching thearc 104 to the support arm 102. The variable array 106 is then mountedto the arc 104 and a position may be selected anywhere about thehemisphere by rotating the support arm 102 about the rod 100, slidingthe arc 104 with respect to the support arm 102, or by moving thevariable array 106 along the arc 104. At any of these points, thesurgeon may be reasonably confident that he has located a point that iscentered about the target area.

While the present invention has been illustrated by the description ofan alternative embodiment, and while the embodiments have been describedin considerable detail, it is not the intention of the applicant torestrict or in anyway limit the scope of the appended claims to suchdetail. Additional advantages and modifications will readily appear tothose skilled in the art. For example, both MRI and CT images may beloaded into computer 30 and coregistration between them may be achievedprior to coregistration with the patient. This provides the surgeon withthe organ details from the NMR scan and the coordinate accuracy of theCT scan. The invention in its broadest aspects is therefore not limitedto the specific details, representative image system and illustrativeexamples shown and described. Accordingly, departures may be made fromsuch details without departing from the spirit or scope of applicant'sgeneral inventive concept.

What is claimed is:
 1. A stereotactic system comprising:an imagingdisplay system for displaying images; an image fiducial selector coupledto said imaging system for selecting fiducials on an image displayed onsaid display system; a target selector coupled to said imaging systemfor selecting a target on an image displayed on said display system; anarticulated arm terminating with a probe and coupled to said imagingsystem, said articulated arm providing spatial coordinates for saidprobe with reference to said imaging system so that a positionassociated with said probe is displayed on said displayed image; apatient fiducial selector coupled to said imaging system and saidarticulated arm for selecting fiducials on a patient that correspond tosaid fiducials selected by said image fiducial selector; acoregistration processor for coregistering said selected patientfiducials to said selected image fiducials so that said coordinatesprovided by said articulated arm may be matched to said displayed imageswhereby a path from said displayed probe position to said selectedtarget may be displayed on said displayed image; and a holder separatefrom the articulated arm and located at a desired position in proximityto a patient, said holder supporting said probe and being pivotable atsaid desired position with respect to said patient to allow a surgeon toevaluate and establish a desired path as displayed on said displayedimage between said desired position and said selected target, saidholder being lockable to fix said holder supporting said probe at adesired orientation directing said probe along said desired path, andthereafter, upon removing said probe, said holder adapted to receive aninstrument, said holder providing independent of said imaging system,support and guidance for said instrument so that said instrument followssaid desired path.
 2. The system of claim 1, said image fiducialselector providing operator selection of said fiducials on saiddisplayed image.
 3. The system of claim 2, said image fiducial selectorfurther comprising:a menu presenting a predetermined number of imagefiducial point identifiers; and an activation icon for selectivelyactivating a selected image fiducial point.
 4. The system of claim 1,said patient fiducial selector further comprising:an operator activatedselector for identifying a patient fiducial, said operator activatedselector enabling said image display system to accept coordinate datafrom said articulated arm and probe to identify a patient fiducial. 5.The system of claim 1 further comprising a patient support adapted tosupport a patient.
 6. The system of claim 5 wherein said holder includesa ball and socket mechanism.
 7. The system of claim 5 wherein saidholder further includes a stereotactic system for selectivelypositioning the probe and the instrument proximate the patient.
 8. Thesystem of claim 7, said stereotactic system comprising:an arc carrierrod, said arc carrier rod being mountable within said holder so thatsaid rod points to a selected target within a patient's head; a supportarm rotatably mounted about said arc carrier rod; and an arc slidablymountable to said support arm so that said arc defines a circle centeredabout said selected target within said patient's head.
 9. Thestereotactic apparatus of claim 8 wherein the holder further includes acollar connectable to the arc at different locations, the collar havinga receptacle for selectively supporting the probe and the instrument ata second selected position and orientation, thereby defining a secondpath between the second selected position and the target point withinthe patient to facilitate an evaluation and operation along the path.10. The stereotactic apparatus of 7 wherein the stereotactic system isremovably attachable to patient at the desired position.
 11. The systemof claim 10, said stereotactic system comprising:a skull ring formounting to a patient's head; and a transfer plate having a receptaclelocated therein for receiving said probe.
 12. The system of claim 11,said holder further comprising a ball adapted to fit within saidreceptacle, said ball being lockable within said receptacle.
 13. Thesystem of claim 5 wherein said holder comprises:a first member locatedat the desired position; a second member supported by and movable withrespect to the first member and selectively supporting the probe and theinstrument; and a clamp disposed with respect to the first and secondmembers for locking the second member in the desired orientation. 14.The stereotactic apparatus of claim 13 wherein the patient supportcomprises a head support.
 15. The stereotactic apparatus of claim 14wherein the head support further comprises a skull clamp.
 16. The systemof claim 5 further comprising a base coupled between the opposite end ofthe articulated arm and the patient support.
 17. The stereotacticapparatus of claim 16 wherein the patient support has a first connector,and the base has a connector connectable with the first connector on thepatient support.
 18. The stereotactic apparatus of claim 17 wherein thebase further includes a releasable clamp for receiving and holding ashaft connected to the opposite end of the articulated arm.
 19. Thestereotactic apparatus of claim 18 wherein the base further includes abore for receiving and holding the probe to facilitate a probeinitialization process.
 20. The stereotactic apparatus of claim 17wherein the base and the first connector have a key system so that thebase is connected to the first connector in a predetermined orientation.21. The system of claim 1, said image display system furthercomprising:a display of said probe and articulated arm position so thatoperation of said articulated arm and probe may be verified.
 22. Thesystem of claim 1, wherein said coregistration processor implements aniterative algorithm for coregistering said selected patient fiducialswith radiological data used to generate said displayed radiologicalimages.
 23. The stereotactic apparatus of claim 1 wherein thearticulated arm further comprises encoders operatively connected to thearticulated arm for providing the spatial coordinate representations ofthe probe.
 24. The system of claim 5 wherein the holder is adjustablysupported with respect to the patient support to allow the holder to bemoved in proximity to the patient.
 25. The system of claim 24 whereinthe holder comprises:a first arm adjustably supported with respect tothe patient support; and a receptacle mounted to one end of the firstarm and movable to the desired position with respect to the patient, thereceptacle selectively receiving the probe and the instrument andholding the respective probe and instrument at the desired orientation.26. The system of claim 25 wherein the holder further comprises a secondarm coupled to the first arm to provide relative motion therebetween,the second arm being connected to the patient support.